Fourier Transform Spectroscopy of the A3Π–X3Σ− Transition of OH+

The OH+ ion is of critical importance to the chemistry in the interstellar medium and is a prerequisite for the generation of more complex chemical species. Submillimeter and ultraviolet observations rely on high quality laboratory spectra. Recent measurements of the fundamental vibrational band and previously unanalyzed Fourier transform spectra of the near-ultraviolet A3Π-X 3Σ- electronic spectrum, acquired at the National Solar Observatory at Kitt Peak in 1989, provide an excellent opportunity to perform a global fit of the available data. These new optical data are approximately four times more precise as compared to the previous values. The fit to the new data provides updated molecular constants, which are necessary to predict the OH+ transition frequencies accurately to support future observations. These new constants are the first published using the modern effective Hamiltonian for a linear molecule. These new molecular constants allow for easy simulation of transition frequencies and spectra using the PGOPHER program. The new constants improve simulations of higher J-value infrared transitions, and represent an improvement of an order of magnitude for some constants pertaining to the optical transitions. © 2017. The American Astronomical Society

Quantitative determination of lineshape parameters from velocity modulation spectroscopy

Velocity Modulation Spectroscopy (VMS) has stood as the gold standard in molecular ion spectroscopy for 30 years. Whether in a traditional uni-directional experiment or more complicated cavity-enhanced layouts with additional layers of modulation, VMS remains the preferred ion detection scheme and is responsible for the detection and transition frequency determination of around 50 molecules. Despite its success, VMS still has a great deal of untapped potential. There have only been two other published studies\footnote{H. Gao \textit{et al.}, Acta Phys. Sin. \textbf{50}, 1463 (2001).}\footnote{S. Civi\v{s}, Chem. Phys. \textbf{186}, 63 (1994).} of VMS lineshapes and both struggle with the highly correlated parameters: linewidth, intensity, and velocity modulation amplitude, \textit{i.e.} the maximum Doppler shift during a period of the discharge. Due to this difficulty, both Gao and Civi\v{s} made concessions to achieve a good fit. Careful analysis of the contour of the transition profile allows us to properly disentangle those parameters in order to probe the environment of the positive column. We can extract the precise values for the translational temperature of the ion, the relative transition intensity, the ion mobility, and the electric field strength just from the lineshape of a single transition. A firm understanding of the lineshape will facilitate chemical and physical investigations of positive columns and allow for a better understanding of more complicated detection schemes

Using NICE-OHVMS lineshapes to study relaxation rates and transition dipole moments

Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy (NICE-OHVMS) is a successful technique that we have developed to sensitively, precisely, and accurately record transitions of molecular ions.\footnote{K.N. Crabtree \textit{et al.}, Chem. Phys. Lett. \textbf{551}, 1 (2012).} It has been used exclusively as a method for precise transition frequency measurement via saturation and fitting of the resultant Lamb dips. NICE-OHVMS has been employed to improve the uncertainties on \chem{H_3^+}, \chem{CH_5^+}, \chem{HeH^+}, and \chem{OH^+}, reducing the transition frequency uncertainties by two orders of magnitude.\footnote{J.N. Hodges \textit{et al.}, J. Chem. Phys. \textbf{139}, 164201 (2013).}\footnote{A.J. Perry \textit{et al.}, J. Mol. Spectrosc. \textbf{317}, 71 (2015).}\footnote{A.J. Perry \textit{et al.}, J. Chem. Phys. \textbf{141}, 101101 (2014).}\footnote{C.R. Marcus \textit{et al.}, Astrophys. J. \textbf{817}, 138 (2016).} Because NICE-OHVMS is a saturation technique, this provides a unique opportunity to access information about the ratio of the transition dipole moment to the relaxation rate of the transition. This can be done in two ways, either through comparison of Lamb dip depth to the transition profile or comparison of the absorption intensity and dispersion intensity. Due to the complexity of the modulation scheme, there are many parameters that affect the apparent intensity of the recorded lineshape. A complete understanding of the lineshape is required to make the measurements of interest. Here we present a model that accounts for the heterodyne modulation and velocity modulation, assuming that the fundamental lineshape is represented by a Voigt profile. Fits to data are made and interpreted in order to extract the saturation parameter

ELUCIDATING THE COMPLEX LINESHAPES RESULTING FROM THE HIGHLY SENSITIVE, ION SELECTIVE, TECHNIQUE NICE-OHVMS

The technique Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy, or NICE-OHVMS, has been used to great effect to precisely and accurately measure a variety of molecular ion transitions from species such as H$_3^+$, CH$_5^+$, HeH$^+$, and HCO$^+$, achieving MHz or in some cases sub-MHz uncertainty.footnote{J.~N. Hodges, textit{et al.} emph{J. Chem. Phys.} (2013), textbf{139}, 164201.}footnote{A.~J. Perry, textit{et al.} emph{J. Chem. Phys.} (2014), textbf{141}, 101101.} It is a powerful technique, but a complete theoretical understanding of the complex NICE-OHVMS lineshape is needed to fully unlock its potential. vspace{1em} NICE-OHVMS is the direct result of the combination of the highly sensitive spectroscopic technique Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy(NICE-OHMS) with Velocity Modulation Spectroscopy(VMS), applying the most sensitive optical detection method with ion species selectivity.footnote{K.~N. Crabtree, textit{et al.} emph{Chem. Phys. Lett.} (2012), textbf{551}, 1-6.} The theoretical underpinnings of NICE-OHMS lineshapes are well established,footnote{F.~M. Schmidt, textit{et al.} emph{J. Opt. Soc. Amer. A} (2008), textbf{24}, 1392--1405.} as are those of VMS.footnote{J.~W. Farley, emph{J. Chem. Phys.} (1991), textbf{95}, 5590--5602.} This presentation is the logical extension of those two preceding bodies of work. Simulations of NICE-OHVMS lineshapes under a variety of conditions and fits of experimental data to the model are presented. The significance and accuracy of the various inferred parameters, along with the prospect of using them to extract additional information from observed transitions, are discussed

Improved Ultraviolet and Infrared Oscillator Strengths for OH+

Molecular ions are key reaction intermediates in the interstellar medium. OH+ plays a central role in the formation of more complex chemical species and for estimating the cosmic ray ionization rate in astrophysical environments. Here, we use a recent analysis of a laboratory spectrum in conjunction with ab initio methods to calculate infrared and ultraviolet oscillator strengths. These new oscillator strengths include branch dependent intensity corrections, arising from the Herman–Wallis effect, that have not been included before. We estimate 10% total uncertainty in the UV and 6% total uncertainty in the IR for the oscillator strengths

Improving Cavity Enhanced Spectroscopy of Molecular Ions in the Mid-Infrared With Up-Conversion Detection and Brewster-Plate Spoilers

The performance of sensitive spectroscopic methods in the mid-IR is often limited by fringing due to parasitic etalons and the background noise in mid-infrared detectors. In particular, the technique Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy (NICE-OHVMS), which is capable of determining the frequencies of strong rovibrational transitions of molecular ions with sub-MHz uncertainty, needs improved sensitivity in order to probe weaker transitions. In this work, we have implemented up-conversion detection with NICE-OHVMS in the 3.2 - 3.9 μm region to enable the use of faster and more sensitive detectors which cover visible wavelengths. The higher bandwidth enabled detection at optimized heterodyne frequencies, which increased the overall signal from the H43 cation by a factor of three and was able to resolve sub-Doppler features which had previously overlapped. Also, we demonstrate the effectiveness of Brewster-plate spoilers to remove fringes due to parasitic etalons in a cavity enhanced technique. Together, these improvements reduced the instrument\u27s noise equivalent absorption to 5.9 X 10-11 cm-1 Hz-1/2, which represents a factor of 34 improvement in sensitivity compared to previous implementations of NICE-OHVMS. This work will enable extended high-precision spectroscopic surveys of H43 and other important molecular ions

Toward Increased Utilization of Historical Hurricane Chronologies

The record of past tropical cyclones provides an important means to evaluate the hurricane hazard. Historical chronologies are a source of information about tropical cyclones prior to the modern era. Chenoweth (2006) describes an archive of 383 tropical cyclones occurring during the eighteenth and nineteenth centuries, largely before the official hurricane record. The present study demonstrates a novel way this archive can be used to articulate historical tropical cyclone activity across space. First, an event in the archive is assigned a series of latitude/longitude coordinates approximating the descriptive locations of the cyclone’s affect. Second, tropical cyclones from the modern record that approach these locations (modern analogs) are mapped. Third, a probable pathway and a realistic track of the archived event is created by averaging the modern analog tracks. As an example, the procedure is used to generate a map showing the tracks of the Atlantic tropical cyclones of 1766. Sensitivity of the methodology to changes in event location and event timing are considered. The study shows that historical hurricane chronologies when combined with a history of cyclone tracks can provide useful information about the older events that is not directly related to where the original information was gathered. When this information is available for all cyclones it should help climatologists better understand long-term variations in tropical cyclone activity

Dynamically Driven Evolution of the Interstellar Medium in M51

Massive star formation occurs in giant molecular clouds (GMCs); an understanding of the evolution of GMCs is a prerequisite to develop theories of star formation and galaxy evolution. We report the highest-fidelity observations of the grand-design spiral galaxy M51 in carbon monoxide (CO) emission, revealing the evolution of GMCs vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (giant molecular associations (GMAs)) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H_2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamics—their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the interarm region and into the next spiral arm passage

Photodetachment study of the 1s3s4s ^4S resonance in He^-

A Feshbach resonance associated with the 1s3s4s ^{4}S state of He^{-} has been observed in the He(1s2s ^{3}S) + e^- (\epsilon s) partial photodetachment cross section. The residual He(1s2s ^{3}S) atoms were resonantly ionized and the resulting He^+ ions were detected in the presence of a small background. A collinear laser-ion beam apparatus was used to attain both high resolution and sensitivity. We measured a resonance energy E_r = 2.959 255(7) eV and a width \Gamma = 0.19(3) meV, in agreement with a recent calculation.Comment: LaTeX article, 4 pages, 3 figures, 21 reference

Dynamically Driven Evolution of the Interstellar Medium in M51

We report the highest-fidelity observations of the spiral galaxy M51 in CO emission, revealing the evolution of giant molecular clouds (GMCs) vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (so-called GMAs) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamics --their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the inter-arm region and into the next spiral arm passage.Comment: 6 pages, including 3 figures. Accepted, ApJ